A thin film electrode or a thin film heating resistor to be used in various electronic components has conventionally been obtained by vapor phase deposition, such as electron beam deposition and sputtering. On the other hand, a method using organometallic compound (metal oxide deposition (MOD)) or a paste method has mainly been used for preparation of a relatively thick conductive film as an electrode or a heating resistor. The former techniques involve such problems as expensiveness of equipment and difficulties in controlling the film composition and in obtaining a large-area film, while the latter techniques have problems of film properties and film thickness.
In the paste method, which is advantageous in initial cost, control of film composition, and ease of obtaining a large-area film, a paste composition comprising a noble metal powder, e.g., Pd, Ag, Pd--Ag or Pt, as a conducting material, a binder, a solvent, glass frit, etc. is applied to a substrate and baked to form a thick film of the noble metal. However, a noble metal, though resistant to oxidation, is very expensive. It has been proposed to form a film of conductive oxide, such as BaPbO.sub.3, by a paste method as disclosed in JP-A-61-225711 (the term "JP-A" as used herein means an unexamined published Japanese patent application), but, as stated above, an oxide film prepared by a paste method is unsuitable for electronic devices (elements) requiring a thin film.
A nonvolatile memory using a thin film of a ferroelectric substance involves fatigue of the ferroelectric substance with switching by metal electrodes, such as Pt. In recent years, it has become known that an oxide electrode suppresses fatigue of a ferroelectric thin film with switching. For example, J. Lee, et al., Appl. Phys. Lett., vol. 63, p. 27 (1993) reported the inhibitory effect of YBa.sub.2 Cu.sub.3 O.sub.x on switching fatigue of Pb(Zr.sub.0.52 Ti.sub.0.48) However, it is not easy to prepare a thin film of such a superconducting material as YBa.sub.2 CU.sub.3 O.sub.x in controlling the oxygen concentration.
As reported in K. Nashtmoto, et al., Mater. Lett., Vol. 10, (7, 8), p. 348 (1991), the present inventors found that a single crystal of LiNbO.sub.3 epitaxially grows on a sapphire single crystal (Al.sub.2 O.sub.3) as a substrate to form a ferroelectric thin film if the starting organometallic compounds, LiOC.sub.2 H.sub.5 and Nb(OC.sub.2 H.sub.5).sub.5, are used without being hydrolyzed beforehand. In some detail, with addition of water to an ethanol solution of a ferroelectric substance precursor, i.e., Li[Nb(OC.sub.2 H.sub.5).sub.6 ], the oriented LiNbO.sub.3 film turns polycrystalline film on baking. If the precursor is baked with no water content, the LiNbO.sub.3 thin film shows epitaxial growth at a temperature as low as 400.degree. C. However, this method has never succeeded in obtaining an epitaxial or oriented film of a conductive oxide.
Further, the present inventors previously obtained a structure in which extremely chemically stable MgO is made to grow by epitaxy on a semiconductor substrate to form a buffer layer and an epitaxial or oriented ferroelectric thin film is formed on the buffer layer. However, they have not yet succeeded in forming an epitaxial or oriented conductive oxide film on the above-mentioned MgO buffer layer.
A thin film of a ferroelectric oxide is expected to be widely applicable to surface elastic wave elements, infrared pyroelectric elements, acoustic optical elements, and electro-optic elements as well as nonvolatile memories for its many excellent properties as a ferroelectric substance, such as ferroelectric properties, piezoelectric properties, pyroelectric properties, and electro-optic effects. In devices using a thin film waveguide structure, preparation of a single crystal thin film is essential for minimizing optical loss and for obtaining polarization characteristics and electro-optic effects as obtained from a single crystal. To this effect, an epitaxial ferroelectrtc thin film of BaTiO.sub.3, PbTiO.sub.3, Pb.sub.1-x La.sub.x (Zr.sub.1-y Ti.sub.y)O.sub.3 (PLZT), LiNbO.sub.3, KNbO.sub.3, Bi.sub.4 Ti.sub.3 O.sub.12, etc. has been formed on a single crystal oxide substrate by Rf-magnetron sputtering, ion beam sputtering, laser ablation, metal oxide chemical vapor deposition (MOCVD), or the like technique.
For integration with semiconductor elements, it is required to form a ferroelectric thin film on a semiconductor substrate. However, epttaxial growth of a ferroelectric substance directly on a semiconductor substrate is difficult due to the high temperature for growth, mutual diffusion between a semiconductor and a ferroelectrtc substance, and oxidation of a semiconductor.
For these reasons, it is necessary to form a capping layer as a buffer layer on a semiconductor substrate, which allows a ferroelectric substance to grow epitaxially thereon at a low temperature, accelerates epitaxial growth of a ferroelectric thin film, and also acts as a barrier against diffusion. While the refractive index of a ferroelectric substance is generally smaller than that of GaAs, a buffer layer having a still smaller refractive index, if obtained, would make it possible to confine semiconductor laser light within a ferroelectric thin film waveguide, thereby making it possible to prepare a light modulation element on a semiconductor laser or to prepare an optical integrated circuit on an Si semiconductor integrated circuit.
Along this line, the present inventors previously proposed (100) epitaxial growth of MgO on a semiconductor (100) substrate (U.S. patent application Ser. No. 07/798,672, now abandoned filed on Nov. 26, 1991 and Japanese Patent Application No. 319228/92). Taking BaTiO.sub.3 on GaAs for instance, the structure thus prepared has a crystallographic relationship of BaTiO.sub.3 (001)//MgO (100)//GaAs (100) as to BaTiO.sub.3 on GaAs, in plane orientation of BaTiO.sub.3 [010]//MgO [001]//GaAs [001].
A thin film electrode and a thin film heating resistor to be used in various electronic components are generally made of metals. However, thin films of Al or Cr are susceptible to oxidation, while thin films of noble metals, e.g., Pd, Ag or Pt, are expensive though resistant to oxidation. JP-A-4-182393 refers to growth of a ferroelectric thin film on RuO.sub.2, etc. However, the lattice constant of RuO.sub.2 has poor conformity to that of an ABO.sub.3 type ferroelectric substance so that it is difficult for a ferroelectric thin film to grow epitaxially on RuO.sub.2.